I breed tropical fish as a hobby. So i built, in my outbuilding with 1 foot deep stone walls) a room where i partitioned off a 4ft wide section which i have now filled with aquariums.

I heat the room to 24 degrees so that i dont need to heat each tank. The room is at a constant temperature of 24 degrees thanks to a thermostat controlling two tube heaters you'd find in a conservatory.

The room has no vents as i wish to keep all the heat in!

The humidity sat between 80-99%.

I had custom made lids added to the aquariums to avoid ANY water evaporation. I have a 6mm hole for a 6mm tube going in and a 22mm hold for a 22mm pipe going in. So it should all be perfect.

I then put on my dehumidifier which took the humidity down to 50% which is fine. But it seemed to be on quite a lot even after the humidity came down.

I switched it off, then within 30mins the humidity had shot back up to 70%! But i do not understand why....where is the water coming from?

THe other issue is the heat that it produces is huge. It heated the whole room up to 28 degrees!

Questions

1) Why is my humidity still high when i have removed the oportunity for water to evaporate?2) Any suggestions as to how to keep the heat down whithout venting?

Only answer to 1) i can think of is that there is a lot of moisture still in the walls?

you need to vent the room,it needs to breathe the build up of heat is causing condensation on the walls. A couple of vents near the top of the walls 100mm in diameter with a sliding hit n miss plastic on the inside should stop it.